There is an opportunity for Boris Johnson to become Tory leader before the next election if David Cameron cannot find an election-winning plan in the next 12 to 18 months. That is the big precondition. Boris will always be a big roll of the dice; a gamble. But a party facing defeat will be prepared to roll the dice.

If there's a demand for Boris he needs to overcome three hurdles in order to meet it. All three are, to coin a phrase, highly jumpable.

Becoming an MP (again)

I don't see this as tricky at all. Just last week one MP – who currently represents one of the safest Conservative seats in the country – explained to me, in some detail, how they would prepare the way for Boris to win selection in their seat and then resign for him at a time of his choosing. I can't imagine the MP who confided in me will be the only one. Ideally, of course, it will be a London seat.

Leaving London

Boris Johnson has, of course, promised to serve a full term as London Mayor. This, for me, is the hardest hurdle for him to overcome but far from insuperable. He doesn't have to make any move for 12 to 18 months, by which time he'll be halfway through his term. Alex Salmond governed Scotland while retaining his seat in parliament (until 2010). Ken Livingstone was briefly MP and Mayor. By the time of the due date for the next election Boris will be three-quarters through his term. If a Mayoral by-election is held on general election day it will be a lot cheaper.

He can use the next 12 to 18 months to do the big foreign trips and batting-for-London exercises while all of the time keeping his distance from what looks likely to be an increasingly fractious coalition. When he makes his move back into the Commons, with further achievements under his belt, he makes this kind of justifying announcement:

"Since I became Mayor of London I have frozen council tax; I have fought for Crossrail and other big investment projects for this city; I did my best to help deliver one of the greatest ever Olympic Games; I have fought for the City of London as well as for a living wage for everyone in London's public sector. I have done all I can with the powers I have. I'm going into Parliament in order to fight for two more changes that London needs and that I cannot deliver from City Hall.

Firstly I am going to fight to end the unfairness of the way that money earned in London is spent in other parts of the country regardless of need. I support distribution from the better off to those without but the current arrangement doesn't achieve that. Some of the poorest communities in the UK are found in London, in rural Wales and in our great northern cities. But the formula for distributing money across the UK sends a disproportionate amount of money to, for example, Scotland. It's not targeted on the poorest parts of Scotland but is available for Edinburgh to use as it wishes. The Coalition has refused to scrap the so-called Barnett formula even though the man who invented it over thirty years ago thinks it's out-of-date. I want to go into parliament to ensure it's replaced with a mechanism that will ensure the poorest communities of Britain, not least in London, get the benefit of what will amount, over time, to hundreds of millions of pounds.

Secondly, I want the next London Mayor to have more powers. I only have a fraction of the powers that my equivalents in New York, Paris and Berlin enjoy. My successor should have extra powers and my first task on entering parliament will be to campaign to introduce those extra powers. London is the greatest city on earth and I want it to have one of the most powerful mayors too. Making that happen will be my final gift to my successor."

Winning over Tory MPs

Boris has enemies in the Commons, some of them motivated by jealousy but many by genuine worries about the Mayor's seriousness. Boris should use the next 12 to 18 months to do all he can to reassure MPs. He will have an early opportunity to prove his credentials in a forthcoming address to a 1922 Committee gathering of Tory MPs and peers. He also needs to work on a manifesto. At the moment – whether it's the EU or the 50p tax rate or education reform – he has more postures than policies. Opinion polls do suggest he'd be a modest improvement on Cameron. That bounce might be bigger in a year's time if this Coalition's cuts bite and growth doesn't resume in adequate measure. Moreover, what current opinion polls don't measure is the bigger bounce that he'll provide if he can deliver a pact with UKIP on a referendum or can outline a tax cut package for the low-paid. "Boris plus blue collar conservatism" will win any Tory leadership contest. The Tory who can win in London (twice) might even win a Tory majority across the whole of the UK.